THE DETECTION OF RESIDUAL PECTIN METHYLESTERASE ACTIVITY IN PASTEURIZED TOMATO JUICES

The enzyme pectin methylesterase (PME) is believed to be involved in t he destabilization and cloud loss of vegetable juices through the de-e sterification of pectin followed by the successive coprecipitation of the pectate with insoluble materials present in the juices. Cloud dest abilization is often observed even when the vegetable products have be en subjected to thermal treatment to produce sterile products and also to stabilize the cloud. Therefore it is possible that loss of cloud m ay be because of residual enzymatic activities surviving the thermal t reatments. However, so far no evidence demonstrating the existence of residual PME activity in pasteurized juices has been published. In thi s paper it is reported that in industrial tomato products showing clou d loss residual PME activity is present. It has been possible to detec t this very low activity by an affinity chromatography procedure. The method is based on a cyanogen bromide-activated resin which is coupled with a pectin methylesterase inhibitor protein purified from kiwi fru it. This resin binds native PME with high selectivity and the enzyme c an be concentrated from the product in a single step. Thus, the very l ow PME residual activity present in pasteurized juice and generally no t detectable with common techniques can be detected and easily determi ned with the method described in this paper.